Slit-shaped micropores

We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3]. This ACF has considerably uniform slit-shaped micropores without mesopores, showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon. Then, ACF has an aspect similar to the regular mesoporous silica in particular in carbon science. Consequently, we can understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. 

According to Ro et al.17 gels made from low water content sols contain residual organic groups, caused by incomplete hydrolysis, which contributes to the formation of micropores during the thermal treatment. Acid-catalyzed gels show slit-shaped micropores and have a fibrous or plate-like structure. Base-catalyzed gels have cylindrical pores and spherical particles. 

As we have seen, the apparent density of He at 4.2 K in the slit-shaped micropores of some activated carbons appears to be c. 0.20gem"3. However, the true density must depend on both the nature of the adsorption system and the pore size and shape. Setoyama and Kaneko (1995) have given a possible range of 0.20-0.23 g cm"3 for the density of helium in the micropores of activated carbons. 

It has been proposed that the slit-shaped micropores may be regarded as spaces between (111) planes in the freshly formed MgO structure. Thus, if the width of each micropore was equivalent to four (111) planes, its width would be 0.96 nm, which is quite close to the hydraulic values. It is not surprising to find pore widening to have occurred as the decomposition neared completion. 

Due to their disorder, activated carbons have a large number of inherent slit-shaped micropores. 

Figure 10.20 Structure model of different carbon materials, (a) nongraphitic carbon exhibiting inherent slit-shaped micropores, (b) multiwall carbon nanotube (MW-CNT),...

More polar adsorbents (such as most oxides) are not easily amenable to a similar procedure because in polar liquids they give rise to specific interactions contributing to the enthalpy of immersion and modifying - in an a-priori unknown manner -its relationship with the surface area. Thus specific interactions can not be the same with the two walls of a slit shaped micropore containing just one molecule. 

Recent research activities on nanoporous materials have stimulated fundamental studies on adsorption mechanism in micropores [1 5]. Both of the precise measurement of high resolution adsorption isotherms from the low P/Po region and molecular simulation showed the presence of monolayer adsorption on the micropore walls and further filling in the residual spaces after monolayer completion for supermicropores (0.7 nm < pore width w <2 nm) the contribution by the monolayer to the filling in the residual spaces is comparable to that by the pore walls [6-10]. Systematic researches on activated carbon fiber (ACF) having slit-shaped micropores[l 1,12] have contributed to elucidation of the mechanism of micropore filling to develop better adsorbents in adsorption and separation engineering. 

R. F. Cracknell, D. Nicholson and N. Quirice, Direct Molecular Dynamics Simulation of Flow down a Chemical Potential Gradient in a Slit-Shaped Micropore, Phys. Rev. Lett. 74 (1995) 2463-2466. 

Microporous solid selected in this study is alumina pillared montmorillonite (Al-PILM) which exhibits well-defined slit-shaped micropores. Four mesoporous solids are examined Two silica MCM-41 samples prepared with quaternary ammonium surfactants dodecyltrimethylammonium bromide (the main carbon chain of the ammonium has 12 carbon atoms. Cl2) and cetyltrimethylammonium bromide. They are labeled as MCM-41 (Cl2) and MCM-41 (Cl6), respectively. The other two are commercial porous silicas (Kieselgel 60 and silica gel 40 A from Aldrich). 

The Nj adsorption isotherms at 77 K were of Type I. The adsorption isotherms of Nj were analyzed by the SPE method using the high resolution a, -plots, as shown in Figure 1. The adsorption isotherm of N, on nonporous carbon black (Mitsubishi Chemical Co. 32B) was used as the standard isotherm. The features of the a -plots were similar to that published in the preceding paper." We can determine the micropore volume W , total surface area a , and the external surface area from the a -plots. The average pore width w can be evaluated from both the surface area and pore volume of slit-shaped micropores. Table 1 summarizes these pore parameters. 

In the Dubinin-Stoeckli (DS) method, a Gaussian pore size distribution is assumed for 7(B) in Eq. (39), based on the premise that for heterogeneous carbons, the original DR equation holds only for those carbons that have a narrow distribution of micropore sizes. This assumption enables Eq. (39) to be integrated into an analytical form involving the error function [119] that relates the structure parameter B to the relative pressure A = -RT ln(P/Po)-The structure parameter B is proportional to the square of the pore halfwidth, for carbon adsorbents that have slit-shaped micropores. 

Kaneko, K., Shimizu, K., and Suzuki, T. (1992). Intrapore field-dependent micropore filling of supercritical N2 in slit-shaped micropores. J. Chem. Phys., 97, 8705-11. 

In addition, depending on the size of the adsorbate molecules, especially in the case of some organic molecules of a large size, molecular sieve effects may occur either because the pore width is narrower than the molecules of the adsorbate or because the shape of the pores does not allow the molecules of the adsorbate to penetrate into the micropores. Thus, slit-shaped micropores formed by the spaces between the carbon layer planes are not accessible to molecules of a spherical geomehy, which have a diameter larger than the pore width. This means that the specific surface area of a carbon is not necessarily proportional to the adsorption capacity of the activated carbon. Pore size distribution, therefore, is a factor that cannot be ignored. 

Relationship between Slit Shape Micropore and Adsorption Energy... 

In Sections 6.10-2 to 6.10-5, we have dealt with cases of interaction between a species and a lattice plane, a slab, two parallel lattice planes and two parallel slabs. Here, we will extend to the case of two parallel lattice planes with sublayers underneath each lattice layer. This case represents the case of activated carbon where the walls of slit-shaped micropore are made of many lattice layers. Although real micropore configuration is more complex than this, this configuration is the closest to describe activated carbon micropore structure. Before we address molecular interacts with two lattice layers with sub-lattice layers underneath, we consider first the interaction between one atom or molecule with one lattice layer with sub-lattice layers. 

The analysis of the cases 4, 5 and 7 are utilised in the study of adsorption isotherm of a nonpolar adsorbate in a microporous solid having slit-shaped micropores, such as activated carbon. To complete the potential theory analysis, we now deal with solids having cylindrical pores of molecular dimension. 

Relationship between slit shape micropore and adsorption energy 282... 

In this section, we briefly introduce a mathematical configuration of a model micropore, the potential energy of adsorbate-adsorbate interaction, and the adsorption potential energy in micropores of different configurations. Then we demonstrate two major applications based on the concept of adsorption potential energy in slit-shaped micropores (1) characterizing the pore size distribution (PSD) of activated carbon and (2) predicting the adsorption equilibria. 

Many researchers [124-128] recognized the role of the micropore size distribution (MPSD) in the study of adsorption equilibria on activated carbon, The MPSD model [126] is based on the idea that adsorption energy heterogeneity is induced by structural heterogeneity, which can be characterized by the size distribution of the slit-shaped micropores. The MPSD is an intrinsic property of activated carbon, and in physical adsorption it dictates the adsorption equilibria through the dispersive interaction between adsorbate and the microporous network of activated carbon. 

For the physical adsorption of a nonpolar adsorbate on activated carbon, the adsorbate-adsorbent interaction is much stronger than the adsorbate-adsorbate interaction. This interaction potential is represented by Steele s 10-4-3 potential, and the adsorbate-adsorbent interaction energy can be taken as the negative of the potential energy minimum inside the slit-shaped micropore. This concept was first proposed by Jagiello and Schwarz [124,125] to characterize the MPSD of activated carbon, and later it was refined and found other applications [127,128]. Although this method does not take into account the adsorbate-adsorbate interaction, which... 

Finn and Monson [139] first tested the predictability of IAS theory for binary systems using the isothermal isobaric Monte Carlo simulation on a single surface. However, this system does not represent real adsorption systems. Tan and Gubbins [140,141] conducted detailed studies on the binary equilibria of the methane-ethane system in slit-shaped micropores using the nonlocal density function theory (NLDFT). The selectivity of ethane to methane was studied in terms of pore width, temperature, pressure, and molar fractions. 

Matsumoto, A., Zhao, J-X., and Tsutsumi, K. 1997. Adsorption behavior on slit-shaped micropores. Langmuir 13 496-501. 

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